Extensive particle size reduction of wheat bran increases the broiler's caecal fermentative capacity, but not bran fermentability ex vivo.

The inclusion of fibre-rich fractions such as wheat bran in broiler feed can increase the production of short-chain fatty acids (SCFA) in the broiler's gut, improving performance, gut health and immunity. Benefits of fine fibre additions (< 300 µm) on caecal fibre fermentation in broilers have been reported, but it is unclear if further reduction in bran particle size continues to stimulate caecal fermentation, and if the ideal particle size for such fermentation is driven by its effects in the gastrointestinal tract or by microbial size preference. To separate these effects, the caecal inocula of 198 Ross 308 male broilers (d 21 and d 35) which received a 0.5% dietary inclusion of 452 µm bran (WB452), 27 µm bran (WB27) or no bran (control) were combined with the same bran fractions in a 3 × 3 ex vivo fermentation simulation design (n = 8). Gas production, pH, SCFA, branched-chain fatty acid (BCFA) content and total bacteria count were measured to assess the effect of bran particle size on the fermentation of the added bran, the fermentative capacity of the bran-fed caecal inocula and their interaction. Both sizes of added bran increased SCFA content (P < 0.05) and lowered the pH (P < 0.05) and tended to increase SCFA-to-BCFA ratio (P < 0.10) and gas production (P < 0.10), while bran particle size had little effect on these parameters during the ex vivo fermentation. The size of the in vivo-fed bran altered the fermentative capacity of the caecal inocula, based on decreased total gas production (P = 0.010) and increased acetic acid content (P = 0.005) for the WB452-fed inoculum, in contrast to tendencies towards a higher total bacteria count (P = 0.075) and SCFA-to-BCFA ratio (P = 0.059) for the WB27-fed inoculum. This shows that bran size does not directly control microbial fermentation but mainly affects the fibre-fermenting capacity of the caecal inocula through its presence in vivo. The lack of an interaction effect between the bran additions and the bran-fed inocula indicates that bran size does not play a major role in priming the fibre-fermenting microbiome. These results highlight the importance of selecting fibre size in feed additions to stimulate broilers' caecal fibre-fermenting capacity.